π/4-x+π/4+x=π/2 cos（π/4-x)cos(π/4+x)=sin（π/4+x)cos(π/4+x)==1/2sin（π/2+2x)
=1/2cos2x=√2/6 cos2x=√2/3 sin2x==√7/3
cos（π/4-x)cos(π/4+x)=根号2/6(
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